Most engines of trucks commonly used in the trucking industry now utilize fuel injectors to deliver an atomized fuel charge to the engine cylinders. Art electronic timing circuit delivers precisely timed electrical pulses for operation of the fuel injector. Such pulses are used in a solenoid stator assembly to reciprocate a solenoid plunger mounted in the fuel injector which controls the injection of fuel into the associated engine cylinder.
The solenoid stator assembly commonly requires a housing to protect its electrical components and to locate them precisely in relation to the reciprocating solenoid plunger. Commonly, such housings have involved insulative plastic housing components surrounding a stator core. The stator core extends through a stator coil which is pulsed with the electrical current to generate the magnetic forces necessary to reciprocate the plunger. In the design of such stator assemblies, it is necessary to overcome severe difficulties created by the very harsh working environment in which the assembly must function.
The stator assembly must be able to accommodate wide variations in operating temperature, from cold start up at below zero temperatures to under the hood temperatures in hot desert conditions exceeding 200.degree. F., causing significant thermal expansion and contraction of the housing components. Leaking fuel droplets under high pressure from the injector can create high pressure within the interior of the plastic housing leading to cracking of the plastic. Over time and under these conditions of inter vibration and fluctuating temperature, plastic components of a housing can develop cracks and hairline fractures. Moreover, the plastic housing components can eventually become embrittled. Also, plastic components are at risk of impact damage if persons servicing the engine accidentally impact them with wrenches or other tools during the course of working on adjacent structures. Fuel injectors under current conditions of operation operate a fuel injection pressures of the order of 2000 pounds per square inch pressure. Escape of fuel under such high working pressures from leaking mechanical portions of the fuel injector can direct extremely high pressure of fuel against the plastic stator housing. The entry of such fuel into a crack in the plastic tends to expand and increase the crack and can cause eventual failure of the housing.
Various types of solenoid stator assembly have been developed to address these problems. One such stator assembly is shown in U.S. Pat. No. 5,155,461 to Teerman et al. for "Solenoid Stator Assembly for Electronically Actuated Fuel Injectors and Method of Manufacturing Same," owned by Diesel Technology Corporation. The Teerman patent discloses an actuator assembly for use with a fuel injector of the same type that the present invention is intended to function with. It has an E-shaped stator core having outer and central pole pieces received within a plastic housing that is bolted to a mounting base on the fuel injector. To prevent passage of leakage fuel under high pressure between the stator core and the housing, the Teerman device incorporates T-shaped notches in the outer faces of the stator pole pieces, into which the plastic material of the housing is molded, to present a barrier against the passage of fuel. The Teerman device is constructed by a process which involves prestressing the outer pole pieces of the core outwardly before the housing is molded about it. The prestressing provides restorative forces to oppose additional, fuel pressure related, forces that might be applied to the outer pole pieces and inhibit additional displacement.
While the T-shaped slots in the outer pole pieces of the Teerman device may be effective to resist fuel migration, it may require an additional machining step to provide such T-shaped slots thereby contributing to the manufacturing cost of the stator core. Additionally, the need to prestress the outer pole pieces before molding the housing around the stator core requires additional process steps during the manufacturing process. Finally, the use of a plastic housing which is directly vulnerable to accidental impacts from mechanical objects and to direct impingement by high pressure of fuel leaves the housing susceptible to the types of problem generally noted above.